1. Field of the Invention
The invention, in general relates to an arrangement for electrophoretically enameling of articles made of metal and, more particularly, to an arrangement employing the so-called two coat one fire method including a process bath for applying an undercoat enamel coat, at least one subsequent rinsing bath ahead of a process bath in which a finish enamel coat is applied before at least yet one further rinsing bath and, finally, a processing furnace. The invention relates, furthermore, to a method of enameling by the above mentioned arrangement.
2. Discussion of the Prior Art
Enameling of metallic components in an electrophoretic enameling plant (ETE) by the two coat one fire method is generally known. As a rule, such a plant is equipped with two processing baths, several rinsing baths and a processing furnace. Enamel undercoating is carried out in a first processing bath. This is followed by two rinsing baths, with slip physically adhering to a component being rinsed off in the first processing bath. The slip is returned to the first processing bath by way of an enamel recovery unit. After another rinse in a second rinsing bath, a finish enamel coat is applied in a second processing bath. This is usually followed by at least one rinsing bath prior to both enamel coats being fired in the processing furnace.
During deposition of the enamel undercoat in the first processing bath, the slip component which produces the wrap-around is also deposited. The quantity of enamel deposited is generally proportional to the current strength and, hence, to the current density. However, in the case of irregularly shaped work pieces, the current density is not uniform over their entire surfaces. Thus, deposition of the finish enamel coat is rendered particularly difficult in those areas in which the current density (edges and curves) is highest, and in those areas more enameling defects is likely to occur.
A two coat one fire method of enamel coating in white and light colors is known from German patent 3,707,401 utilizing a special undercoat enamel frit which must have very particular properties. The disadvantage of such a method is that only a very particular composition of undercoat enamel frit can be used. Individual percentages of the components have empirically to be determined beforehand. Composing the frit is very complex, and yet in the problem areas of the work piece to be coated, enameling defects still cannot be avoided.
German Patent 3,626,424 discloses a method of electrophoretically enameling Al-free layers of enamel. The undercoat enamel is applied at a coating thickness of 80-120 .mu.m, and the specific conductivity of the finish enamel coat is set at value below 2,000 .mu.S/cm.sup.-1. This method can only be practiced in connection with catalytic finish enamel which in contrast to white enamel constitutes a non-melted mixture of raw materials. Because of this, the application of the finish enamel coat is simpler.